peter wrote:

Isn't it possible for a neighbor to throw something in the toilet that
eventually clogs the city sewer line? If this happens, then subsequent
flushing or use of water would cause a backup. Or is the city sewer
line so large that nothing passing through a toilet could possible
get stuck there?

See this recent story:
http://www2.sunjournal.com/html/doug...yid=248787&t=3

You have to study the picture carefully